UAF researchers finds dead salmon nourish lakes

ANCHORAGE (AP) -- A University of Alaska Fairbanks researcher has found evidence that decaying sockeye salmon nourish lakes in Alaska and lead to better salmon production.

The findings by Bruce Finney, published in the journal Science on Friday, have broad implications for management of fishing in Alaska.

Finney, 43, was able to use a particular isotope of nitrogen to log salmon returns to the lakes for the last 300 years. He concluded that commercial fishing can reduce the salmon ''fertilizer'' deposited in lakes and potentially cut future productivity.

''I think the data show that in some systems, you could be robbing from the future to some extent,'' Finney said in an interview Thursday.

But climatic changes also have an unmistakable impact, he said. ''There's certainly no quick and easy formula.''

Karluk Lake on Kodiak Island was one of the lakes Finney studied in research he began in 1993.

In that lake, he found nutrients from salmon carcasses typically provide more than half the phosphorus and nitrogen that support plankton populations. Those plankton, in turn, provide food for salmon fry.

Karluk was a huge producer of salmon before commercial harvests began in 1882.

''Karluk Lake used to be the most productive lake in the world for its size,'' Finney said.

Total sockeye production at Karluk averaged more than 2 million fish until about 1910, and some years reached 4 million. But by the 1970s, returns were down to 500,000 fish or less.

While climatic changes had something to do with the decline, Karluk didn't recover when Bristol Bay production boomed in the 1980s and early 1990s.

''Our data suggest that the prolonged 20th-century collapse of the Karluk sockeye fishery was driven in part by reductions in carcass-derived nutrients from overharvest,'' Finney wrote in the Science report. ''The sediment data infer a positive feedback system, in which higher adult salmon abundance leads to increases in nutrient loadings.''

That, in turn, leads to more salmon production.

Not all lakes get their nutrients primarily from returning salmon, Finney said, noting that salmon carcasses don't play nearly the same role in fertilizing the lakes that produce Bristol Bay salmon.

Recent record catches in those systems would suggest that any disruption of the nutrient cycle had a minor impact on production, he said.

Core samples show nitrogen levels in lake-bottom sediment remained constant in the Bristol Bay lakes Finney studied, even though catches rose significantly.

''That's because management philosophy is to manage for a constant, optimal number of spawning salmon in each system,'' Finney said. ''So, essentially they harvest the excess salmon, and the nitrogen levels in the sediment remained relatively constant.''

Finney is going beyond the 300 years of data compiled so far, a record that takes up about 18 inches of sediment on the lake bottom.

''We're working on records that go back thousands of years in some places,'' he said. ''This winter, we're going to be coring some of these lakes back to the last Ice Age.''

The earth has been significantly cooler in recent centuries, so the earlier data will provide information on salmon productivity in warmer times, Finney said. The data could help answer questions about global warming.

Besides Karluk, Finney also took samples from Frazer, Red and Akalura lakes on Kodiak Island, and from Ugashik, Becharof and Tazimina lakes near Bristol Bay on the Alaska Peninsula.

Jon Sweetman of UAF and three Canadian researchers are co-authors of the Science article.